I don’t know what you’ve heard or from whom. The 722.3 transmission behind a 603 is solid when properly maintained. 200-250K miles might be their lifetime. The 722.6 transmission behind a 606 turbo can go longer. I don’t know if these 722.6s have an iffy bushing like gasser 6s or a roller bearing like gasser V8s and 6s from 2000. The 722.6 can be more particular about maintenance and fluid level.

The 606 turbo has more electronics and throttle by wire that can be tedious to tansplant but you can swap in a 603 IP for mechanical control. The 603 IP would have to be modified to flow the fuel levels of the original 606 IP. 603 doesn’t have an O2 sensor.

The 603 only has a MAF for emission controls. Unlikely you’d carry these over in a swap so no MAF to worry about. 603 has no TPS or VSS for engine control or brake signal for the transmission.

You seem pretty down on these drivetrains to be thinking of a swap. I would think an original MB application is as reliable as the drivetrain can get. Once you take it out of its element, reliability can only suffer.

Sixto
98 E320 wagon
02 C320 wagon